This invention relates to pressure transducers and more particularly to an economical pressure transducer employing piezoresistive transducers and adapted to respond to a differential pressure.
The piezoresistive sensor has achieved widespread use as a pressure or force measuring element. In general, the prior art is replete with a number of patents which describe various configurations and methods of fabricating the piezoresistive device. Presently, there is an increasing demand for the use of pressure transducers in various industries. As such, these devices have to be reliable, while relatively economical. For example, in the automotive industry, pressure transducers are employed to measure the pressure of various engine fluids or to measure the pressure at the manifold. In any event, the piezoresistor is an attractive configuration in regard to such uses.
Basically, the piezoresistive device can be fabricated by conventional integrated circuit capability and hence, is compatible and can be made using the same techniques as are being presently employed for the manufacture of microprocessors and other integrated circuits which are being employed in automobiles and for other commercial operations, as well.
The automobile industry and many other industries as well require an absolute pressure sensor. Such a device has an output which is always referenced to vacuum. Differential pressure measurements can be accomplished by a differential transducer which provides an output which is the difference between two pressures. In the case of a gage sensor one of these pressures is atmospheric pressure and the other pressure is the pressure being monitored. Essentially, a gage transducer produces an output which is the difference between atmospheric pressure and the monitored pressure. In the case of the absolute pressure transducer, the output is truly indicative of monitored pressure.
While the use of piezoresistive sensors in such applications is desirable, there presents a number of problems associated in employing such devices. A piezoresistive device, as indicated, usually consists of a bridge pattern of resistors which are mounted or diffused on one side of a relatively thin diaphragm member. The diaphragm which may be fabricated from silicon, deflects upon applications of a pressure thereto causing the piezoresistors to vary their magnitude according to the deflection of the diaphragm. In regard to such devices, there is one side of the device which contains the resistors which must be exposed to a first pressure. The other side of the diaphragm is also exposed to a pressure.
In the case of an absolute transducer, it is extremely difficult using standard economical integrated circuit wafer fabrication and packaging techniques to form a reference pressure on the circuit side of the sensing diaphragm. The monitored pressure is best applied to the backside of the diaphragm or side not containing sensing circuitry and leadwires in order to provide best protection from deleterious atmospheres for instance those containing various types of engine pollutants and other substances which would effect the sensors and or other circuitry on the front of the diaphragm.
It is therefore an object of the present invention to provide an absolute pressure transducer which is capable of operating in contaminated environments and simple to construct. Such devices can be mass produced as lending themselves to integrated circuit construction techniques and based on such techniques, are economical and reliable.